One of the major changes which many pregnant women experience is largely due to an alteration in the center of gravity of their body which shifts because of a gain in weight and the change in the circumference of the torso. During pregnancy the natural curves in the backbone become more pronounced as the body's weight increases and its center of gravity moves forward in relation to the spine. Increased stress causes the pelvis to tip forward and the posture to be poor which commonly results in fatigue and backache since the back muscles are forced to do work for which they were not designed. The more S-shaped the spinal column becomes the more these muscles are taxed. Pregnancy further jeopardizes the vulnerable pelvic floor because of the increased weight it must support as the uterus enlarges. Like a bow string, the back muscles tend to shorten as the abdominal muscles lengthen. Further, during pregnancy, as a result of the shift of the center of gravity, the pregnant woman tends to stand further back on her heels. Further compensation may occur such as bringing the head and shoulders forward, raising the lumbar curve or swaying back from the waist. Backache commonly results from this incorrect weight distribution since the muscles of the back are now doing extra work.
Although it has been recognized that comfortable shoes which provide generally adequate support are especially important as the pregnancy advances, the kinds of shoes which are actually worn by pregnant women frequently contribute to the problem rather than alleviating it. For example, high heel shoes throw the pelvis and body weight forward, which hollows the back and strains ligiments in the hip and knees. Because of this unstable position, the muscles have to work much harder and the weight on the toes may cause the arches and balls of the feet to experience discomfort. Other styles of shoes, for example with the negative heel, stretch these muscles even more than flats and frequently become uncomfortable.